MATEC Web of Conferences 33 , 0 6 0 0 2 (2015)
DOI: 10.1051/ m atec conf/ 201 5 33 0 6 0 0 2
C Owned by the authors, published by EDP Sciences, 2015
Electrochemical characterization of oxidized nanostructured superelastic
Ti-Nb-Zr alloy for medical implants
1,a
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1
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Yulia Zhukova , Yury Pustov , Vadim Sheremetyev , Anton Konopatsky , Mikhail Filonov and Vladimir Brailovski
1
National University of Science and Technology “MISIS”, Leninskiy prosp. 4, Moscow 119049, Russian Federation
École de Technologie Supérieure, 1100, Notre-Dame Street West, Montreal (Quebec), Canada H3C 1K3
2
Abstract. Metastable Ti-Nb-based shape memory and superelastic alloys are known to be strong candidates for bone
implant applications. The issues of the materials’ biochemical and biomechanical compatibility and its
characterization are reviewed. Thermomechanical treatment is conventionally applied to these alloys in order to
obtain supreme functional properties; the processing scheme comprises cold rolling and post-deformation air
annealing. The structure and electrochemical characteristics of annealing-induced oxide films were studied by
scanning electron microscopy, open circuit potential measurement and voltammetry. It is shown that the samples after
the annealing treatment exhibit higher steady-state potential value and lower anodic dissolution current density in
simulated biological solution, compared with the samples with mechanically removed oxide films. At the same time,
the samples with thermal oxide films exhibited lower rate of passive layer recovery than those subjected to
mechanical renewal of the oxidized surface. This fact underlies the recommendation to remove the annealing-induced
oxide film from the implants operating under friction conditions.
1 Introduction
One of the major research areas in biomedical materials
science is the development of bone replacement materials
[1–4]. Metals and alloys are the most widely used
materials to manufacture load-bearing implants, including
joint prosthesis, dental implants, etc., because of their
high strength, ductility and machinability.
Despite of the great progress in this field during the
past decades, there is still no perfect material that would
fulfil all the strict requirements for such medical devices.
Generally speaking, an implantable material must be
biocompatible; moreover, from the “bio-mimicking”
standpoint, the structure and properties of a bonereplacing substance should be as close to those of bone as
possible [3,4].
The biocompatibility requirements comprise the
aspects of biomechanical and biochemical compatibility.
Biochemical compatibility implies the absence of
prolonged adverse reactions between host tissues and
implanted material. From this standpoint, pure metals can
be classified as toxic (Co, Cu, Ni, V), capsule (Fe, Al,
Mo, etc.) and vital (Ti, Nb, Ta, Zr, Pt) [5]. In addition to
that, the products of corrosion, wear and other mediumassisted degradation processes must be safe for the
biological environment.
Biomechanical compatibility is the important property
underlying the durability and functional stability of longterm and permanent load-bearing implants. It indicates
that the implant’s mechanical properties and deformation
a
behavior should be close to those of bone tissue [6]. In
particular, the material should have low Young’s
modulus (ideally adjustable to 1–40 GPa targeting at
different types of bone tissue), which would prevent
“stress shielding”, and support large elastic strains (not
less than 0.5 %). Conventional metallic biomaterials like
stainless steel, Co–Cr, as well as pure titanium and major
part of titanium alloys, fail to meet this requirement.
Possible generally accepted solution for this problem
is the development of low modulus titanium alloys
comprised of “vital” elements (Nb, Ta, Zr, with Ti as
balance) and exhibiting shape memory and superelasticity
effect resulting from reversible β to α'' martensitic
transformation [7–12]; Ti-Nb-based alloys in particular
[for example, 13–15].
Conventional metals are spontaneously covered with
oxides (TiO2 in case of titanium) which determine their
chemical stability in aggressive environment. When
metallic materials are implanted in human body media,
initial reactions occur between their surface and the host
tissues. Therefore, the surface oxide films play an
important role not only against corrosion but also in
tissue compatibility.
Metastable Ti-Nb-based alloys exhibit superelastic
behavior if they are subjected to a specific sequence of
thermomechanical treatment (TMT). The final stage of
this TMT comprises air annealing in the 500 to 600 °C
temperature range and results in the formation of
substantial surface oxide films. The aim of the present
study
was
the
preliminary
electrochemical
Corresponding author: zhukova@misis.ru (Dr. Yulia Zhukova)
This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits distribution, and reproduction in any medium, provided the original work is properly cited.
Article available at http://www.matec-conferences.org or http://dx.doi.org/10.1051/matecconf/20153306002
MATEC Web of Conferences
characterization of the oxide layers of Ti-Nb-based
alloys; such data for these materials have not yet been
reported.
(a)
2 Materials and methods
The experimental material was Ti-21.8Nb-6.0Zr (at. %)
alloy produced by vacuum-arc melting with consumable
electrode and hot-forged, in recrystallized condition, after
water-quenching from 900 °C (hereinafter designated as
TNZ0). The samples were further subjected to
thermomechanical treatment leading to the formation of
nanosubgrain substructure with improved functional
properties [8]: cold rolling with a true strain of 0.3,
mechanical polishing down to Ra = 0.05 μm of
roughness, and 1-hour post-deformation annealing in air
at 600 °C with subsequent water quenching. One part of
the samples was mechanically polished to remove the
oxide layer (samples denoted TNZ1); another part was
studied in as-annealed condition (TNZ2).
The morphology of oxidized surface was studied by
scanning electron microscopy (JEOL JSM-6610LV)
using TNZ2 sample polished cross-section. The samples
were mounted in EpofixTM cold-setting embedding resin
and mechanically polished. Before the test, a thin
conducting layer of graphite was deposited on the surface
to form conducting layer.
The immersing medium was Hank’s salt solution
simulating inorganic components of bone tissue;
composition (g l–1): 8 NaCl, 0.4 KCl, 0.12
Na2HPO4·12H2O, 0.06 KH2PO4, 0.2 MgSO4·7H2O, 0.35
NaHCO3, 0.14 CaCl2, H2O (up to 1 l of the solution); pH
= 7.4.
Electrochemical measurements were carried out with
electronic potentiostat IPC-Micro by open circuit
potential (OCP) and potentiodynamic voltammetry
methods (potential scan rate 0.1 mV s–1, starting from
steady-state potential value) at (37 ± 1) °C. Saturated
silver chloride electrode (hereafter SSCE) was used as a
reference electrode, and platinum electrode was used as
an auxiliary electrode. Prior to each experiment, all
samples were cleaned in distilled water for 10 min using
ultrasonic cleaner CT-405.
Due to the fact that the implants performance may be
significantly affected by mechanical loads leading to
local destruction of passive films formed in biological
media, it was considered of practical interest to study the
kinetics of passive layer restoration in the case of
mechanical renewal. To investigate the self-healing
ability of mechanically damaged surfaces in Hank’s
solution, scratch tests were carried out: the samples were
scored with diamond cutter with resulting scratch size of
approximately 0.5 × 3 mm.
(b)
Figure 1. SEM microphotographs of TNZ2 sample crosssection: backscattered electron image (a), secondary electron
image (b).
As seen from Fig. 1 that the annealing-induced oxide
film is about 5 μm thick and features through pores about
1 μm in size. This oxide layer may have strong influence
on corrosion and electrochemical behavior of the studied
alloys, thus electrochemical measurements were carried
out on the samples with (TNZ2) or without (TNZ1)
thermal oxide.
The character of OCP evolution during exposure to
simulated solution is an important tool for the
investigation of tendency to passivation and protective
film formation on the alloy surface.
Fig. 2 shows OCP curves for TNZ samples in Hank’s
solution.
3 Results and discussion
Fig. 1 shows SEM microphotographs of TNZ2 sample
cross-section containing the annealing-induced oxide
film.
Figure 2. OCP curves for Ti-21.8Nb-6Zr alloy samples in
Hank’s solution.
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It can be seen from Fig. 2 that the OCP values of both
the thermomechanically treated and untreated samples
shift towards higher values, which is an indication of
surface self-passivation.
The steady-state potential value of TNZ1 sample is
markedly higher than that of TNZ0. This tendency is
presumably the result of the nanosubstructure formation
in Ti-Nb-Zr alloys during TMT [8]. It is well known that
nanostructure formation decreases corrosion resistance
when a material corrodes (dissolves) in active mode.
However, Ti-based alloys are known to undergo
spontaneous self-passivation, and this process is
intensified by the surface activation process, which is
underlined by the developed network of subgrain
boundaries and dislocations; such an increase in
corrosion resistance for nanostructured pure titanium has
already been reported, for example, in [12].
It is of crucial importance that in case of thermal
oxide films, their steady-state OCP is 300 mV higher
(TNZ2) than that of removed films (TNZ1). The observed
OCP shift during the initial period of exposition is
presumably related to the clogging of oxide film pores
with the resultants of the electrolyte - bare metal reaction.
In order to validate the observed tendency for selfpassivation, anodic polarization curves were measured
after the establishment of a steady-state OCP (Fig. 3).
surface ability in the case of a natural (not thermal)
passive oxide layer.
Figure 4. OCP curves for Ti-21.8Nb-6.0Zr alloy after TMT in
Hank’s solution after mechanical renewal of the sample surface
(corresponding time moment indicated with arrow).
In accordance with the methodology for Ti-Nb-based
alloys described in [13], the comparative tendency to
passivation in simulated solution can be estimated using
kinetic regularities of OCP evolution during exposition.
The right part of OCP curves in Fig. 4 (after the
surface renewal indicated by arrow) was normalized, i.e.
nondimensionalized in 0 – 1 limits. The first part on the
processed curves e (t) was approximated by the
logarithmic equation e = a + k ln (t + c), where k, a and c
are coefficients, with k being the kinetic constant of oxide
film growth. The values of the coefficients along with the
correlation coefficient for TNZ1 and TNZ2 samples are
listed in Table 1.
Table 1. Logarithmic approximation results for OCP curves of
TNZ1 and TNZ2 samples after mechanical renewal of the
sample surface.
Sample
Figure 3. Polarization curves of Ti-21.8Nb-6.0Zr alloy in
Hank’s solution.
It can be seen from Fig. 3 that the polarization curves
lack activation peaks, which is coherent with the OCP
measurements, and suggests the tendency to selfpassivation. It should be noted that TNZ2 samples
dissolve two times slower during anodic polarization in
200 – 400 mV range than TNZ1 samples. However, TNZ1
samples exhibit broader potential range of strong
passivity up to the potential of transpassivation or anodic
oxygen evolution (~ 1100 mV).
Fig. 4 shows OCP curves of TNZ1 and TNZ2 samples
in the case of mechanical renewal of the sample surface.
It can be seen that TNZ2 samples after scratching feature
significant OCP drop, which indicates the breakdown of
passive film with subsequent recovery during ongoing
exposition. At the same time, the equal load applied to
TNZ1 samples leads to smaller potential drop with faster
film recovery. This fact indicates higher self-healing
TNZ1
TNZ2
k
a
c
0.139 ±
0.004
0.125 ±
0.003
1.023 ±
0.007
0.745 ±
0.003
0.000 ±
0.000
0.003 ±
0.001
R2
0.98
0.91
It can be seen from Table 1 that the kinetic constants
of the passive film formation rate k are highly dependent
on the presence of thermal oxide on the samples’ surface:
TNZ1 samples exhibit 20 – 30 % higher film formation
rate than TNZ2 samples.
4 Conclusion
The influence of thermomechanical treatment (postdeformation annealing in particular) on the
electrochemical characteristics of nanostructured
superelastic Ti-21.8Nb-6.0Zr (at. %) alloy designed for
medical implants is studied.
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MATEC Web of Conferences
The general trend is that the TMT annealing-induced
micrometer-range oxide films improve the alloys‘
electrochemical properties in Hank’s solution, such as
their steady-state electrode potential and anodic
dissolution current density. However, it can be
recommended to remove the annealing-induced oxide
films when implants are to perform in friction conditions,
because the passive layer recovery for the samples with
the mechanically renewed surface films is faster.
17. Yu.S. Zhukova, Yu.A. Pustov, M.R. Filonov,
Protection of Metals and Physical Chemistry of
Surfaces 48, 315 (2012)
Acknowledgements
The work was carried out with financial support from
the Ministry of Education and Science of the Russian
Federation (project ID RFMEFI57514X0094).
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